1. Field of the Invention
The present invention relates to a carrier providing a charge to a toner by frictionizing the toner, a developer including the carrier and a toner, and a process cartridge and an image forming apparatus using the developer.
2. Discussion of the Background
An electrophotographic image forming method typically forms an electrostatic latent image on a photoconductive image bearer; provides a charged toner to the electrostatic latent image to form a visual image; transfers the visual toner image onto a transfer medium such as papers; and fixes the visual toner image on the transfer medium with a heat, a pressure or a solvent vapor, etc.
The electrophotographic image forming method is broadly classified to a two-component developing method wherein a toner is charged by mixing the toner with a carrier and a one-component developing method wherein a toner is charged without using a carrier.
The one-component developing method is broadly classified to a magnetic developing method and a non-magnetic developing method according to whether a toner is magnetically borne by a developing roller.
Conventionally, the two-component developing method which has good charge stability and build ability of the toner and stably produces quality images for long periods is mostly used for printers, copiers and complex machines which are required to have high-speed printability and quality image reproducibility; and the one-component developing method is mostly used for small printers and facsimiles which are required to be space-saving and low-cost.
Recently in particular, color images are produced more, and therefore high-quality images and stability of image quality are demanded more than ever.
The two-component developing method stably produces good images by mixing and stirring a toner and a carrier to charge them as mentioned above, but the carrier deteriorates and a mixing ratio between the toner and carrier varies. Since the mixing ratio between the toner and carrier, i.e., a toner concentration affects image density, the toner needs to be supplied as needed to produce stable images. Further, when the toner and carrier are mixed and stirred, a pressure is applied thereto, and therefore a surface of the particulate carrier is abraded and the toner is spent thereon, which cause deterioration and instability of friction chargeability of the carrier, resulting in deterioration of image density and quality.
In addition, recently, both copiers and printers are required to save space, and to meet this requirement, a toner including a release agent is used in many cases because a fixer becomes small by saving a fixing oil and applying a small amount thereof. The release agent on the surface of the toner tends to be spent on the carrier.
In addition, since a toner having a small particle diameter has a large specific surface area, a surface energy thereof increases and the toner tends to adhere to neighboring materials, resulting in deterioration of fluidity thereof. When the fluidity of the toner deteriorates, the toner is not smoothly supplied to an image developer, and is not fully mixed with the carrier to sufficiently be charged. Therefore, a large amount of a fluidizer is added to the toner to impart fluidity, and further to impart mixability thereto to be uniformly mixed with the carrier in a short time. When the amount of the fluidizer is large, an existential status thereof changes as time passes, resulting in variation of the fluidity and frictional charge quantity of the toner and carrier.
In addition, a particulate inorganic material is often used as the fluidizer, and such a particulate material abrades other contact members, resulting in acceleration of abrasion of a coated layer of the carrier.
Therefore, the following technologies are disclosed to protect a photoreceptor from being damaged or abraded by the fluidizer, and to control charge polarity and charge quantity thereof.
For example, Japanese Laid-Open Patent Publication No. 2-131250 discloses a two-component developer comprising a carrier and a toner, wherein the carrier has a ceramic coating of a specific metal alkoxide, having a thickness of from 0.05 to 5 μm.
Japanese Laid-Open Patent Publication No. 7-295303 discloses a coating agent for an electrophotographic carrier, including a specific organopolysiloxane.
Japanese Laid-Open Patent Publications Nos. 7-181743 and 7-181744 disclose an electrophotographic carrier coated and hardened with a partial hydrolysis sol formed of at least one of Si alkoxide, Ti alkoxide, Al alkoxide and Zr alkoxide.
Japanese Laid-Open Patent Publication No. 7-181745 discloses an electrophotographic carrier coated and hardened with a partial hydrolysis sol formed of at least one of Si alkoxide, Ti alkoxide, Al alkoxide and Zr alkoxide, and including an inorganic oxide powder having an average particle diameter of from 1 to 100 nm and a hydroxyl group on a surface thereof.
Japanese Laid-Open Patent Publication No. 8-006309 discloses a carrier for developing an electrostatic latent image, having a metal oxide layer including at least one of aluminum, magnesium, zinc and lead on a core material.
Japanese Laid-Open Patent Publication No. 8-030039 discloses a carrier for developing an electrostatic latent image, wherein a particulate core material is coated with a metal oxide formed by a sol-gel method.
Japanese Laid-Open Patent Publication No. 11-202561 discloses a coating agent for a charging member, including a product from a reaction at a specific ratio between a silicone resin having a specific composition and an acrylic resin polymerized with at least one radical polymerizing vinyl monomer as a main component.
Japanese Laid-Open Patent Publication No. 2000-172020 discloses a carrier particles including a chemical compound selected from the group consisting of monomeric polyfunctional organosilanes, hydrolyzates of monomeric polyfunctional organosilanes, reaction products of monomeric polyfunctional organosilanes and heteroatom-containing organosilanes, wherein the reaction products of monomeric polyfunctional organosilanes and alkoxides further includes 5-50 wt. % silicon atom-free polymeric compound.
Japanese Laid-Open Patent Publication No. 2000-162827 discloses a magnetic carrier for an electrophotographic developer, formed of a particulate magnetic powder having an average particle diameter of from 10 to 200 μm including a particulate magnetic core material coated with a resin composition formed of a metallic hardener, a silane coupling oligomer and a silicone resin.
Japanese Laid-Open Patent Publication No. 2001-272826 discloses an electrophotographic carrier prepared by applying the following coating agent on a surface of a core material comprising a magnetic material. The coating agent includes a composite material produced from an organic compound having two or more functional groups, a compound and/or its hydrolyzed condensed product and a compound having a functional group which can react with the functional group of the organic compound and having a functional group which can react with the compound. The composite material consists of two or more kinds of composite components having different contents of nonvolatile components measured by the weight reduction by heating at 900° C. in air.
On the other hand, when a toner has a smaller particle diameter, the toner has a larger specific surface area. Although a charge quantity of the toner per unit area does not change, a charge quantity thereof per unit volume increases in terms of results.
A toner quantity forming an image depends on a toner quantity according to a latent image formed on an image bearer. Therefore, when a charge quantity of the toner per unit volume is too large, a sufficient toner quantity to form an image cannot be prepared and higher image quality with a toner having a small particle diameter is not always realized.
To secure a sufficient toner quantity on the image bearer, there is a method of enlarging a contrast of the latent image. However, in this case, the image bearer needs to have a sufficiently large charge potential, which tends to be disadvantageous in terms of consumed energy, temporal stability and electrostatic fatigue.
The above-mentioned technologies improving surface strength of the carrier and preventing a spent toner thereon cannot be expected at all to have effects on increase of charge quantity of the toner having a smaller particle diameter.
Besides, (1) abrasion of a surface of a carrier, (2) peeling of a surface coated layer of a carrier, (3) breakup of a carrier and (4) toner constituents adhered (spent) onto a carrier cause deterioration of chargeability, gradation from a desired electrical resistance, and tips and abrasion powders of foreign particles, resulting in deterioration of image density and image resolution, foggy background, physical/electrical damage to an image bearer and durability of the carrier. To solve these problems, various suggestions have been made in terms of materials for the carrier.
However, the suggestions cannot be expected to have sufficient effects in an environment wherein a fixing temperature is further lowered and a carrier is expected to have a longer life.
For examples, in Japanese Laid-Open Patent Publications Nos. 8-6308, 9-269614, 9-311504 and 10-239913, a matrix resin alone covers almost all the surface of a carrier, and therefore prevention of adherence of a toner mostly depends on a coverage of the matrix resin over the carrier and sufficient spent prevention is not always activated. When a toner capable of having a low-temperature fixability is used, in a method disclosed in Japanese Laid-Open Patent Publication No. 10-198078, a part of the carrier, having the same component as that of the binder resin of a toner, tends to a base the toner particles adhere to and the toner occasionally has a low charge quantity.
In addition, it has been suggested many times that a silicone resin having comparatively a low surface energy is coated on a carrier. However, the silicone resin does not adhere to a core material of the carrier well due to its low surface energy.
Besides, Japanese Laid-Open Patent Publication No. 58-108548 discloses a carrier coated with a specific resin material, Japanese Laid-Open Patent Publications Nos. 54-155048, 57-40267, 58-108549, 59-166968 and 6-202381, and Japanese Patent Publication No. 1-19584 disclose a carrier coated with a specific resin material further including various additives, and Japanese Patent No. 3120460 discloses a carrier coated with a specific resin material, which an additive adheres to. Japanese Laid-Open Patent Publication No. 8-6307 discloses that a benzoguanamine-n-butylalcohol-formaldehyde copolymer is used as a main component of a carrier coating material, and Japanese Patent No. 2683624 discloses that a crosslinked product between a melamine resin and an acrylic resin is used as a carrier coating material. Still, these carriers do not have sufficient durability.
Japanese Laid-Open Patent Publications Nos. 2001-117287, 2001-117288 and 2001-188388 disclose that a thermoplastic resin is used as a coating resin, and a coated layer including a particulate material larger than a thickness of a binder resin.
Japanese Laid-Open Patent Publication No. 9-319161 discloses a method of dispersing a specific thermosetting particulate resin in a matrix resin, wherein a carrier has a coated layer having a property equivalent to that of the initial coated layer even after abraded. However, the abrasion itself is not sufficiently decreased. Japanese Patent No. 2998633 discloses a method of dispersing an electroconductive particulate powder together with the specific thermosetting particulate resin in a matrix resin. However, the abrasion itself is not sufficiently decreased, either.
Because of these reasons, a need exists for a carrier suitable for producing high-quality images, having stable properties without overcharging a toner for long periods.